Large format arrays of imaging detectors have an increasing need to exhibit high sensitivity and low dark current, while operating under very low light level conditions. In addition, many imaging systems require detectors with a broad spectral range, facilitating large focal plane arrays that can cover a wide range of the spectrum in a single detector type.
Conventional imaging devices are illuminated from the front surface of the device, where the front surface is normally defined as the surface that has metal and gate structures used for manipulating charge. Such structures are necessary for the imaging device to function. However, the structures include relatively thick materials which correspond to undesired effects such as absorbing and scattering incident light and/or particles. The front surface gate structures normally use insulators (e.g. thermally grown silicon oxides) which are transparent and relatively benign for light, but absorb low energy particles and deep UV light.
The front surface gate structures also include gates, normally made of polycrystalline silicon, which have stronger absorption. Additionally, the gate structures include metals, which absorb and scatter strongly, but are designed to cover only a small portion of the surface. One way to avoid unpleasant effects of the front surface structures is to turn the imaging device over and illuminate from the back surface. In this way, light can theoretically get into the silicon detector, without having to go through the front surface. The process of illuminating a device from the back surface is called back illumination.
For most silicon detectors, back illumination immediately runs into a problem that is related to the materials used in making the device. Most devices use a relatively pure silicon for the light-sensitive region of the detector. This is normally accomplished by starting with a wafer including a substrate on which has been grown an “epilayer.”
The “epilayer” is an epitaxially grown layer of high quality silicon. Except in the special case of high purity silicon detectors, the substrate is generally highly-doped. In most instances, but not all, the substrate is p++ silicon, while the epilayer includes p− silicon (e.g. doped at a few times 10^14 dopants per cubic centimeter). In general, the highly doped substrate is at least an order of magnitude thicker than the epilayer, and the substrate absorbs most if not all of the incident light.
Thus, simply turning the detector over does not work because the substrate absorbs all of the light. One way to minimize the absorption is to remove the substrate using a thinning process. The thinning process exposes the back surface of the epilayer to incident light, so that light may get into the detector. Unfortunately, using the thinning process to expose the back surface of the epilayer is not enough.
After light and/or particles get into the detector, electron-hole pairs are formed, and it is the electron-hole pairs that are detected. Shallow penetrating radiation generates electron-hole pairs near the surface, and deep penetrating radiation generates electron-hole pairs through a much larger volume of the detector. Often, the internal structure of the detector prevents collecting charge generated by the shallow penetrating radiation. The observation that shallow penetrating radiation (e.g. photons or particles) is more difficult to detect than deeper penetrating radiation such as red light is sometimes referred to as a “dead layer.”
After the substrate is removed using the thinning process, a bare silicon surface is left. The silicon is relatively pure because it is part of the epilayer. This purity allows free electrons and holes to have much longer lifetimes in pure material than in highly doped material. However, using relatively pure silicon creates a problem for the as-thinned detectors.
Silicon surfaces have surface-related defects. These defects are electrically active as they are able to trap charge in “mid-gap” states. These mid-gap states cause many problems. For example, the mid-gap states are rapidly filled by charge from the underlying silicon and a surface charge is generated. With a p-type epilayer, the surface gains a positive charge, and with an n-type epilayer, the surface gains a negative charge.
This charge creates an electric field inside the semiconductor, which penetrates into the semiconductor a certain distance before it is finally shielded by charge in the silicon lattice. The region in which this electric field exists is called the “backside potential well.” Such backside potential well causes many problems for the detector.
First, the polarity of the charge is such that the backside potential well traps all of the signal charge that is to be detected. Second, the signal charge can contribute to the shielding properties of the semiconductor such that the backside potential well is unstable. In other words, the depth to which the electric field penetrates into the silicon varies with illumination history, temperature, and, unfortunately, the environment. Thus, even deeper penetrating radiation, which the detector can still see, is detected with an efficiency that changes with time and prior illumination. This phenomenon is called quantum efficiency hysteresis.
To solve these problems, something has to be done to the back surface to “passivate” the defects or to prevent the bad effects from occurring. There are various ways of passivating the back surface, including ion implantation/laser anneal and flash gate processes. In addition, delta-doping may be utilized.
Detectors with p−− epilayers, and n-type buried channels, are collecting the electrons in “electron-hole pairs.” However, some detectors collect the holes in “electron-hole pairs.” Such detectors are made with an n−− substrate, with a p-type buried channel. One such detector is the high purity CCD developed by Lawrence Berkeley National Laboratory (LBNL), for astrophysics applications.
Infrared sensitivity in silicon detectors is limited by the fact that infrared light penetrates so deeply into the silicon detector that some of the light is transmitted through the entire thickness of the detector and is lost. To avoid this problem, a thicker detector may be utilized to absorb more of the infrared light. However, thicker detectors include mainly “field-free” silicon. In other words, charge generated far from the front surface of the detector is shielded from fields generated by front-surface electrodes by the thick silicon. Thus, the deeper signal charge may simply diffuse around and end up anywhere.
Charge diffusion leads to fuzzy images, which is undesirable in scientific detectors. To fix that problem, a method is used for depositing an electrical contact on the back surface of the detector, and then applying a voltage to bias the detector into a condition known as “full depletion.” A fully-depleted detector has an electric field throughout the entire thickness of the device. In a fully-depleted detector, charge generated anywhere in the detector is driven by an electric field toward the collection well near the front surface, where it is supposed to go. In this way, a 250 micron detector with 10 micron pixels may begin to approach high resolution imaging.
Full-depletion is useful for thick, high purity detectors, but it requires an electrode on the back surface. The electrode has to have certain properties in order to work well such as adequate conductivity, transparency (i.e. the electrode should not absorb or scatter light), and a low “dark current” (i.e. not a significant source of “dark current”), etc.
A process was developed for making the back surface electrode out of “in situ doped polysilicon” (ISDP). With some development, electrodes were able to be made with acceptable properties. However, the ISDP electrode is deposited at a substrate temperature of about 600° C., which is too high a temperature for a detector with aluminum metallization. At temperatures above 450° C., the aluminum starts to react with the underlying silicon, which destroys the detector.
Therefore, in order to use the ISDP electrode process, a detector manufacturer (e.g. a CCD manufacturer) must interrupt the manufacturing process before the metal layers are deposited. The partially-processed detectors (e.g. CCD wafers) may then be shipped to a location where the detectors may be thinned (e.g. because the original wafers are too thick to be fully depleted) in order to achieve a thickness of about 250 microns. The ISDP layer may then be grown on the back surface, and the metallization and the rest of the manufacturing process may be completed. However, interrupting the detector fabrication process is burdensome and inefficient. There is thus a need for addressing these and/or other issues associated with the prior art.
FIG. 1A shows a method 100 for growing a back surface contact on an imaging detector, in accordance with the prior art. As shown, manufacturing of an imaging device is started. See operation 102. Before the image device is metallized as part of the manufacturing, the manufacturing process is interrupted. See operation 104.
A substrate of the imaging device is then thinned to around 250 microns. See operation 106. A delta-doped layer is then grown on a back surface of the imaging device at a temperature of at least 600° C. See operation 108. Once the delta-doped layer is grown on the back surface of the imaging device, the imaging device may be metallized and the manufacturing of the imaging device may be completed. See operation 110.
Because the delta-doped layer is deposited at a substrate temperature of at least 600° C., the manufacturing of the imaging detector must be interrupted so that, in the case the metallization includes providing an aluminum electrode, the aluminum may start to react with the underlying silicon, destroying the detector. In some cases, the partially processed detectors may be shipped to a location where the detectors may be thinned and the delta-doped layer may be deposited. Once the thinning has occurred and the delta-doped layer is grown, the imaging device is allowed to cool to an acceptable temperature such that aluminum metallization may occur at a temperature of less than 450° C. It should be noted that this may involve shipping the imaging device back to the manufacturer to complete the manufacturing.